Sounds are transmitted through the outer ear to the eardrum which moves the bones of the middle ear and excites the cochlea. The cochlea is a long narrow duct wound spirally about its axis for approximately two and a half turns. The fluid filled cochlea transmits mechanical sound waves in response to received sounds and in cooperation with the cochlear duct, functions as a transducer to generate electric impulses which are transmitted to the cochlear nerves and thence to the brain.
Profoundly deaf patients have lost the ability to transduce the outer mechanical sound wave into meaningful action potentials along the neural substrate of the cochlea. In persons with total sensorineural hearing loss, therefore, the cochlea does not respond to sound waves to generate electrical signals for transmission to the cochlear nerves. An auditory prosthesis for the deaf requires a suitable stimulation electrode capable of stimulating the auditory nerves. A cochlear implant is a neural prosthesis designed to permanently restore the sensation of sound in profoundly and severely deaf patients, including children.
The interface between the prosthesis and the auditory nerve consists of an electrode carrier inserted into the fluid filled scala tympani region of the cochlea. The scala tympani fluid is highly conductive to electrical current. The anatomy of the scala tympani is that of an upright spiraling cone with an inner wall and an outer wall. The center of the spiral is called the modiolar. The modiolar is where the spiral ganglion cells reside. Cochlear prostheses attempt to stimulate the spiral ganglion cells directly with small currents delivered by a multitude of electrodes regularly distributed along the carrier. The stimulating current is synchronized with the environmental sound via complex input output functions and digital signal processing.
The distance between the excitable spiral ganglion cells and their axons, and the electrode carrier is relatively large, up to 2 mm at the basal end of the cochlea. This distance becomes significant as it causes the threshold and maximum currents responsible for the stimulation to be relatively high. Furthermore, with increasing distance, the potential field generated by adjacent electrodes may stimulate an overlapping population of nerve cells, particularly at higher currents. The spatial selectivity of each electrode is reduced. The dynamic range is also lower. The power consumption of the implanted prosthesis is higher.
Fundamental features of a cochlear implant electrode array carrier must include attributes that allow the carrier to be easily implantable, explantable, reimplantable, and biocompatible. In addition, as very delicate tissues line the scala tympani, the insertion process must prove to be as atraumatic as possible. Finally, a last requisite for a perimodiolar electrode involves the device's ability to hug the modiolus whether the array is fully inserted or not despite the unique geometry of the individuals inner ear canal.
Several methods have been proposed to attempt a displacement or an initial positioning of the cochlear implants proximal to the auditory nerve cells. One manufacturer has routinely implanted a space filling, pre-curved electrode introduced with an insertion tool has been routinely implanted by one manufacturer. Unfortunately, the results of this placement have failed to provide a viable option as the electrode, positioned somewhat between the inner and outer wall of the scala tympani, does not establish adequate contact. Theoretical or experimental devices have been proposed based on 1) a bilaminar array with half of the carrier made of a material which can absorb liquid and increases in length (differential expansion may cause the array to curve in an unpredictable manner), 2) an array with an external and parallel polymer forward positioning, 3) an array with a shape memory nitinol core, 4) a preshaped array made straight with bioresorbable material, and 5) an array with active positioning through the passage of current into a nitinol wire. In vitro and in vivo data concerning the placement of the electrode array in such proposed devices is sketchy or absent. In some cases, the insertion and displacement trauma is estimated unacceptable.
The arduous task of displacing a non-space-filling electrode array from the lateral to medial wall of the scala tympani is compounded by the fact that the inner and outer wall of the scala tympani are respectively 40 and 18 mm long. If the array is fully inserted along the outer wall (about 31 mm for a 0.5 mm diameter electrode), then the process of the array hugging the modiolus, is not a simple radial translation from the lateral wall to the medial wall. Movement of the array from the outer wall to the inner wall principally involves a longitudinal displacement of all points on the array in the axial direction of the scala tympani, and from the apical to basal end. A significant length of the electrode array has to be forced out of the scala tympani. If the array is partially inserted along the outer wall, then the process of the array hugging the modiolus may be either a forward or backward displacement of the array. In the case of a forward displacement, because of the spiraling shape of the cochlea, the necessary longitudinal displacement of points on the array to go from the outer wall to the inner wall increases with distance down the scala tympani.
For example, a point located against the lateral wall 12 mm into the scala tympani has to travel up to a point radially facing the 23 mm outer wall mark to embrace the inner wall of the cochlea. The amount of forward displacement increases linearly from base to apex. Furthermore, points located on the first turn past the 12 mm mark on the outer wall, have to move around the narrowing corner to the second turn of the cochlea to hug the inner wall.